1. Field of the Invention
The present invention relates to methods used for positioning a data head over a data track in a rotating disk drive data storage device. More particularly, the present invention relates to methods for positioning a data head over a radially located data track in a disk file which employs a multimodal servo control system wherein the operating mode of the servo control system is determined by a predicted relative data head position.
2. The Prior Art
Disk files are information storage devices which utilize one or more rotatable disks with concentric data tracks formed thereon for magnetically recording data, one or more data heads for reading or writing data onto the various tracks, and an actuator (usually a voice coil motor (VCM)) connected by a support arm assembly to the data head(s) for moving the data head(s) to a desired track and maintaining the data head(s) over the track centerline during read or write operations. Each disk has two data surfaces, and the disks are disposed on a spindle driven by a spindle motor. The read/write (RNV) information is electrically coupled to and from the data head by way of a R/W channel. The RAN channel may include amplifiers, filters and detectors, as required.
The radial movement of the data head to a desired data track is referred to as track accessing, or "seeking", while the maintaining of the data head over the centerline of the desired data track during a read or write operation is referred to as "track following". The VCM typically comprises a coil movable through the magnetic field of a permanent magnetic stator as is well known in the art. The application of current to the VCM causes the coil, and thus the attached data head, to move radially over the surface of the disk. The acceleration of the coil is proportional to the applied current, so that ideally the head is perfectly stationary over a desired track if there is no current applied to the coil.
In disk files which have a relatively high density of data tracks on the disk, for example, more than about 2000 tracks per inch (TPI), it is necessary to employ a servo control system to efficiently move the data head between the data tracks and to maintain the data head within a given tolerance of distance over the centerline of the desired data track during read or write operations. Such a servo system requires that servo information be prerecorded on the disk file. Servo information can be prerecorded on either a dedicated servo surface or on servo sectors located on each disk surface or on a combination of both. In a disk file which has less than four disks, it is generally more efficient to prerecord the servo sector information on each track of every disk surface. In a disk file which has more than four disks, a dedicated servo surface either alone or in combination with servo sector information is generally employed. All of these mechanisms for servo control information are well known to those of ordinary skill in the art.
Servo sectors are angularly spaced pie-piece-shaped sectors which are interspersed among the data sectors on the data disks. The prerecorded servo information is normally written to the disks by a servo writer at the factory. The prerecorded servo information, including servo bursts, is read off the disks, demodulated, and processed by the servo control system. The results are then applied to the input of the servo electronics which control the current to the VCM and thus the radial position of the head(s) over the disk surface.
Servo bursts are fixed amplitude analog signals written to the disk surface at precise locations which are used to define the exact radial position of the data tracks on the disk surface. Prior art servo bursts come in groups of 2, 3, 4 or more. As the head passes over each servo burst, the amplitude of the analog signal read from the servo burst is a function of the radial position of the head and is a maximum as the head passes over the center of the burst. Referring to FIG. 1, a prior art servo sector is diagrammed. In operation, the data head will pass over the AGC portion of the servo sector 12, the servo sync 14 and the index 16 before reaching the servo information. The servo bursts are diagrammed by boxes labelled A, B, C, and D. The data head will read the A burst, the B burst, the C burst and then the D burst, all of which are constant amplitude signals. The centers of data tracks are, by definition, located at the edges of the A/B bursts as shown and labelled in FIG. 1 as T1, T2, T3, etc. In this servo pattern, the servo bursts are recorded in spatial quadrature with one another. The servo bursts A through D are used to generate a position error signal (PES). The PES represents the radial off track distance of the data head from the track centerline within a 1/2 track width distance. The PES provides a position signal which is linear with respect to the actual position of the data head within the data track.
The servo bursts are generally independently magnitude detected in a read channel. Detecting each servo burst independently generates some DC offsets among the bursts which must be calibrated during the drive power up. If the DC offsets are not calibrated the position of the data head determined by the spatial quadrature of the servo bursts will not be correct. A better technique, as is presently employed by the assignee of the inventors herein in the Ministor 1.8" disk drive product line, is to use a single detector for all servo bursts so that there is no offset associated with any channel. Another advantage of using a single detector is that hardware complexity and size is significantly reduced. According to this approach, only one output from the read channel and one sample and hold capacitor for detecting the servo bursts is required instead of the standard four employed for a servo sector having four servo bursts.
In the prior art, servo control systems describe the position of the data head on the disk surface either in relative or absolute terms. These will be referred to as a relative data head position description and an absolute data head position description. In a servo control system using a relative data head position description, the relative data head position is defined as the difference between the current position of the data head and the target position of the data head. In a servo control system using an absolute description, the absolute data head position is defined simply as the current position of the data head.
With the high track densities of modern disk drives, an issue arises as to whether a relative or absolute data head position description should be used. For currently commercially available disk drives, the number of data tracks can be expected to be about 3500 TPI. In a 1.8" form factor drive this results in approximately 1500 tracks across the disk surface. The number of bits required to address 1500 tracks is eleven (2.sup.11 =2048). Additional bits are used to describe the data head position. Of the additional bits, one is usually allocated to sign (this is used in relative position description only and indicates whether movement of the data head to the target requires an increase or a decrease in radial distance to the disk spindle or center) and a selected number of bits are used to describe the PES.
The number of bits used to describe the PES is a critical determination in control systems. If enough bits are not allocated to the PES, then control of the data head cannot be maintained during track following. For the example just cited, if an absolute description is used, then for 16 bit processing, eleven bits will be allocated to the track number at all times, and five bits will be available to describe the PES. It is well accepted by those of ordinary skill in the disk drive art that five bits, which provides a resolution of 1 in 32, is not adequate for the track following function. Further, in all instances known to the inventors, a minimum of 8 bits (resolution of 1 in 256) are used for the PES in track following in prior art disk drives.
The resolution requirements of the PES are due to the dynamics of the disk drive system itself and the fact that the data head must usually be within approximately 10% of the data track width away from the track centerline for write operations. These dynamics with which the servo control system must operate include at least the following: position reference noise, uncertain high-frequency actuator dynamics, sensor noise, sensor nonlinearities, variable actuator parameters which may vary from motor to motor, vary for the length of the stroke, vary for the operating temperature of the drive, and vary for the age of the drive. All prior art disk drive servo control systems account for the dynamics of the disk drive either by mathematical modeling of the disk drive or by some method of lumped parameter control.
If the PES does not have enough resolution, the disk drive dynamics will cause the servo control system to lose control of the positioning of the data head. As such, where an absolute data head position description is used, the current requirements in the art of at least 8 bits for the PES in track following and 11 bits for the track number for current commercial track densities of approximately 2000 TPI or greater, lead to the conclusion that at least 19 bit processing must be employed. Since 19 bit processors are not reasonably commercially available, 32 bit processing is therefore employed.
Where a relative data head position description is used as an alternative to the absolute position description, 16 bit processing is possible, even with current commercial track densities. An example of 16 bit processing using relative addressing can be seen in the Ministor 1.8" disk drive, MP340P, manufactured by the assignee of the present invention. In the Ministor MP340P disk drive, the servo control system is implemented using a proportional integral derivative control method, known in the art as PID control. With PID control, the servo control system determines the control signal for supplying current to the VCM based upon the summation of signals derived from the position of the data head, the derivative of the position of the data head, and the integral of the position of the data head. Proportional control is generated by multiplying the relative data head position by a fixed gain. (It should be noted that the fixed gain by which the position difference is multiplied changes as the difference in position between the current position of the data head and the target position of the data head changes from greater than 1/2 track to less than 1/2 track). The derivative of the position of the data head is used to represent the velocity of the data head. The derivative control is generated by multiplying the data head velocity by a fixed gain, and is used to provide damping for the data head. The integral control is generated by multiplying the integrated relative data head position by a fixed gain, and the integral control is used to increase a low frequency gain which corrects for the error of the position of the data head when it is track following. The summation of the proportional, derivative and integral control is used to determine the current supplied to the VCM. Another example of a servo control system using PID control can be found in the servo control system in the program listing attached to U.S. Pat. No. 5,170,299 granted to Ronald R. Moon and entitled "Edge Servo for Disk Drive Head Positioner." U.S. Pat. No. 5,170,299 and which is hereby incorporated herein by reference.
In the Ministor MP340P drive both a relative data head position description and PID control are used as follows. For a current data head position which is more than 16 tracks away from the target track, the description of the data head position used by the servo control system is given by 8 bits of track information, 7 bits of PES and 1 bit for sign. As the data head moves to within 16 tracks, but is still more than 1/2 track from the target track, the data head position description has 4 bits for the track number, 9 bits for the PES and 1 bit for the sign. For less than 1/2 track, 1 bit is allocated to the track number, 9 bits are allocated to the PES and 1 bit to sign. Accordingly, the requirement of providing a minimum of 8 bits for the PES in track following, as explained above, is satisfied.
It is well known in the art that PID control provides less than optimal control for several reasons. One is that the dynamics of the disk drive system, as explained above, are all treated in a lumped fashion. In other words, none of the different disk drive dynamics is accounted for separately, rather, they are all lumped together. Another weakness in PID control is that the signal representing the integral of the position cannot realistically be included by the servo control system in the control signal until the data head is within 16 tracks of the target position, because it may introduce too much error. Accordingly, in the Ministor MP340P, the servo control system in effect uses PD control until the data head is within 16 tracks of the target position and uses PID control thereafter. Finally, a major weakness of PID control can be seen in comparing PID control to a servo control system which implements control by a method known as the state estimator method.
An example of the state estimator method is given in U.S. Pat. No. 4,679,103 granted to Michael I. Workman and entitled "Digital Servo Control System for a Data Recording Disk File" and which is incorporated herein by reference. In the state estimator method, the dynamics of the disk drive system are more specifically accounted for by constants having values which are derived from parameters which describe a working model of the VCM and support arm assembly for the data head. A detailed description of the design and implementation of the dynamics of such systems is given in Chapter 12 of the book titled "Digital Control of Dynamic Systems" 2nd edition by Franklin, Powell, and Workman. Using these state variables the position, velocity and bias of the data head are predicted. From the predicted position, velocity and bias of the data head and the measured position of the data head and the measured VCM current (those of ordinary skill in the art will recognize that in small disk drives utilizing transconductance amplifiers to supply the current to VCM, that the VCM current need not be measured), estimates of the position, velocity and bias of the data head are made.
These estimates provide for much better servo system control than the measured position, the derivative of the measured position and the integral of the measured position provided by PID control. The absence of using an estimate of the position, velocity and bias of the data head in PID control is a substantial shortcoming. The use of the state estimator method to predict the data head position functions as a filter to minimize the effect of the noise in the measured data head position. It is well known in the art, that in PID control, that differentiating a noisy measured head position to obtain the data head velocity introduces error into the data head velocity and, accordingly, is not an ideal technique. Without the predicted data head position, predicted data head velocity and the predicted data head bias the servo control will respond to these irregularities. In track seeking, this means that the radial velocity of the data head is not optimized to avoid overshoot, resulting in a seeking performance which is poor by the current standards in the art. During track seeking, excessive disturbance occurs when the servo control system responds to error in the measured head position. This disturbance can excite the mechanical resonance of the data head support arm assembly which results in oscillation of the data head while arriving at the desired data track. In servo control systems using an estimated position, much of the error in position imposed by the disk drive dynamics will be filtered out so that the performance of the disk drive may be more readily optimized.
One advantage that PID control has held over the state estimator method is that it may be implemented using a relative data head position description and hence, 16 bit processing as described above. It has generally been accepted in the art that the resolution of the state variables must be such so that during track following the dynamics of the disk drive are correctly modeled. In view of this, the number of bits allocated to the PES in the position descriptions has been a minimum of 8 bits, regardless of the number of tracks between the current position of the data head and the target position of the data head. Accordingly, it has been accepted in the art that 32 bits of processing are required to implement the state estimator method, since at least 11 bits will be allocated to the track number for current disk drives and at least 8 bits will be allocated to the PES.
More generally, it can be stated that in the present approach in the art, that if M bits are required to represent the track numbers on the disk surface, and N bits are required to represent the PES in track following, then at least M+N bits of processing will be required. Table 12.16 on page 746 from "Digital Control of Dynamic Systems" 2nd edition by Franklin, Powell, and Workman, summarizes the required resolution for each state variable and emphasizes the requirement of 32 bit processing. There have been essentially two hardware approaches relevant to the present invention taken in the prior art to implement the 32 bit processing used by servo control systems for the state estimator method.
One approach is exemplified by the Seagate 2.5" products, for example the SGT9550A, which utilize two processors. The first processor is a 32 bit precision DSP processor, the Texas Instruments TMS320C10, which is used to implement the control code to control the actuator of the VCM. The second processor is an 8 bit microcontroller which is used for interfacing the DSP to the disk drive and also interfacing the disk drive to the host system. For smaller form factor drives, such as 1.8" drives, using two processors presents substantial problems: (1) two separate processors require too much space, leaving even less space for other processing circuits, (2) combining the two processors in an ASIC is a very expensive proposition, and (3) using both a stand alone 32 bit precision DSP and a microcontroller is a relatively expensive device solution to use in a commodity-type commercial product ` such as a disk drive where margins are thin.
A second 32 bit processing approach found in the prior art is exemplified by the Western Digital 2.5" disk drive products, for example WDAC 2170. These systems employ a single non-DSP processor, the Intel 196KC for both servo system control and controller interface codes. The Intel 196KC uses 32 bit precision for the servo control code, but the multiplier is about 10 times slower than that found in a typical DSP. This degrades servo controller performance by introducing a longer computation delay, which is defined as the time interval from the servo burst interrupt until the new control signal is sent to the D/A converter. In addition, the data throughput is significantly degraded because the Intel 196KC splits time between the servo control calculation and the interface controller function. This approach results in increased seek time, one of the most critical specifications in a disk drive along with price, capacity and size.
It is well known in the art of disk drive manufacturing that the predominate objectives are to make disk drives which are smaller, have higher capacity, have faster seek times and have a lower cost. As discussed above, these objectives are often at odds with each other. Higher capacity and the increased accuracy of the state estimator method has set the standard for servo control processing at 32 bits which is at odds with size requirements and cost as exemplified by the Seagate 2.5" products or performance as exemplified by the Western Digital 2.5" products.